Devices for filtering biological solutions generally involve three primary components, i.e. a membrane filter interposed between two vessels, a feed container located upstream of the membrane for holding the sample solution to be filtered and a filtrate container located downstream of the membrane filter for collecting the filtered sample solution. Often a vacuum is drawn downstream of the membrane to increase the rate of filtration by creating a pressure differential across the filter. However, in such cases provisions must be made to maintain the pressure differential across the membrane and thus assuring that the filtration will not stop.
The arrangement of components for vacuum filtration can take various forms; however, especially in laboratory settings, ease of use, reduced storage requirements and minimal disposable hardware are important concerns as is avoiding spillage of the biological solution. In certain other applications, preserving the sterility of the solution being filtered is also important.
An example of a vacuum filter device is described in U.S. Pat. No. 4,673,501 wherein an open funnel for receiving a sample of solution to be filtered is arranged to be sealed to the top of a bottle for collecting filtrate. The base of the funnel includes a membrane filter positioned such that when the sample to be filtered is poured into the top of the funnel all of the sample solution is directed to flow through the membrane filter. A vacuum conduit which is adapted to be connected to a vacuum source is formed within the base of the funnel and allows a vacuum to be drawn within the filtrate bottle thereby drawing the sample solution through the membrane filter. Since the pressure differential across the filter is constant due to the application of a vacuum on the downstream side of the filter and atmospheric pressure present on the liquid surface of the open funnel, rapid filtration is possible and any reduction in flow rate is due to filter fouling. Nonetheless, vacuum filter devices of the type described in this patent suffer from a number of drawbacks which make them inconvenient for laboratory use. First, these devices require the liquid sample be transferred from its normal laboratory container to an open funnel which can lead to spills, sample contacting more surfaces which can introduce loses to the surface by non specific binding or addition of unwanted species from improperly cleaned or handled labware. Because of the liquid weight concentrated at the top of this assembly, they are prone to tipping and hence spilling the biological solution during pouring of sample or when connecting the vacuum hoses. Aside from the inconvenience to the user in handling the fluid to be filtered, there is an enhanced risk of compromising the sterility of the particular biological solution due to the open nature of this device. Moreover, the large size of these filter assemblies results in their taking up limited laboratory storage space. In addition, since the containers utilized in the filtration process are disposable and intended for one-time use, a significant amount of solid waste is generated by these filter assemblies and the associated pre- and post-filtration containers.
To minimize the amount of solid waste and fluid transfers, U.S. Pat. No. 5,141,639 describes a vacuum filter assembly wherein the membrane filter is disposed in a cover sealable to the filtrate container. The cover is formed with a feed port in the form of a tubular feed nipple on the upstream side of the membrane filter. A length of tubing is connected at one end to the feed nipple and the other end is directly inserted into a sample container housing the solution to be filtered. The cover also includes a filtrate outlet port and a vacuum port, both of which fluidically connect with the downstream side of the membrane filter. When tubing is attached to the vacuum port and a vacuum is drawn the sample solution to be filtered is caused to flow through the tubing and pass through the membrane filter to the filtrate container. As is the case with the aforementioned U.S. Pat. No. 4,673,501, the pressure difference in this prior art assembly remains constant because of the vacuum in the filtrate container and the atmospheric pressure acting on the liquid surface in the open feed or sample container. While this device minimizes the amount of solid waste generated during filtration, it is cumbersome to use as the operator must assemble the tubing to the cover and hold the over on the filtrate container until the necessary vacuum pressure has been achieved in the filtrate container. Additionally, the feed tubing must be maintained submerged in the sample container to avoid air being drawn into the sample solution which could disrupt the filtration. In addition, the sample is housed in an open container; therefore, the risk of compromising biological integrity or sterility is heightened.
U.S. Pat. No. 5,603,900 overcame many of these problems by providing a device with two closed holders, one for the feed and the other for the filtrate fluid with both attached to a common vacuum filtration body which had a vent upstream of the filter and a vacuum port downstream of the filter. However, it still is limited in applications as the filter is sealed, heat sealed, to the upper inner surface of the body. This limits the types of membranes or filters that can be used in the device to those which can be heat sealed to the body surface. Other filters such as filter paper, ultrafiltration membranes, glass filters, and/or multiple layered filters cannot be used in this system despite the strong need to do so.
Thus it is apparent that the need still exists for an improved vacuum filter device that can accommodate a wide variety of filters and integrally seal them in the body of the filter device.